Method and system for extended volume imaging using MRI with parallel reception

ABSTRACT

A method and apparatus for producing an image from an extended volume of interest within a subject using a Magnetic Resonance Imaging (MRI) system are provided. The method comprises translating the volume using a positioning device along an axis of the MRI system. A plurality of MR signals are detected from at least one radiofrequency (RF) coil array for a given field-of-view within the MRI system as the positioning device is translated. The plurality of MR signals are sent to a plurality of receivers wherein the receivers are each adapted to adjust their respective center frequencies at a rate commensurate with a rate of translation of the positioning device. A plurality of respective sub-images corresponding to the plurality MR signals for each of the plurality of receivers are combined to form a composite image of the volume of interest.

BACKGROUND OF INVENTION

This invention relates to a method and system for medical imaging. Moreparticularly, this invention relates to a method and system for extendedvolume imaging using a Magnetic Resonance Imaging (MRI) system andemploying a moving patient table.

Certain clinical situations require that a head-to-toe scan of a patientbe made. For example, metastatic cancer can occur anywhere in the bodyand patients at risk for metastatic disease need to be evaluatedregularly. Currently head-to-toe or, alternatively, extended volumeimaging is generally performed with whole body positron emissiontomography (PET) systems or nuclear studies in which small amounts ofradioactive substances are given to the patient and allowed to collectin regions of rapid tumor growth. The imaging sensitivity andspecificity of magnetic resonance (MR), however, makes MR a moredesirable choice for diagnosis particularly when Gadolinium (Gd)contrast agents are employed. Gd contrast agents are typicallyadministered intravenously and tend to collect in regions ofangiogenesis associated with active tumors. Unfortunately, MR scannershave limited sensitive volumes and whole body scanning requires that aseries of images be made at different stations. Patient motion and tableregistration can make the joining of these separate images a challengeand tend to create image artifacts referred to as “stitching artifacts”.

Generally, imaging using a MRI system involves imaging a volume ofinterest in a MRI scanner's usable volume. The usable volume is definedas a contiguous area inside the patient bore of a Magnetic Resonancescanner and it can be limited in size. Typically, when the usable volumefails to cover an extended object, a method for examining the wholevolume containing the object employs repeated executions of positioningand imaging a fraction of the whole volume within the scanner's usablevolume to obtain regional images. A subsequent assembling operation thenassembles or “stitches” the regional images together to produce a finalimage of the whole volume of interest. Such an approach is typicallychallenged by the “stitching” artifact issue wherein resulting finalimages often suffer from distinctive artifacts at the boundaries of the“stitched” pieces.

Existing techniques achieve correct combination of regional imagesthrough full spatial encoding along patient table motion direction. Withother existing methods, the patient table is held stationary while datais collected and moved between the collection of the regional images.These techniques minimize “stitching” artifacts by using slab selectionprofiles that are as rectangular as possible, and/or discarding imagedata near the boundaries. As a result, these techniques tend to beinflexible, require prolonged radio frequency (RF) excitation, andinvolve considerable acquisition efficiency degradation.

More recently, imaging an extended volume is performed by means ofsimultaneous patient table translation thereby allowing examination of afield of view that extends beyond the usable volume of an MR scanner. Itis however very difficult to achieve 3-dimensional whole body coveragewith favorable mapping accuracy, spatial resolution, signal-to-noiseratio and total scan time.

Parallel imaging offers a way to speed up conventional MR imaging. Theidea of detecting MR signal with multiple coils receiving in parallelhas been explored. Recent advances represented by simultaneousacquisition of spatial harmonics (referred to as SMASH) and sensitivityencoding (SENSE) make up for a reduced number of gradient-driven spatialencodes by integrating data from an array of receive RF coils. SMASH andthe likes assume a frequency perspective they fill up skipped k-spacelines through approximating Fourier harmonics with linearly combinedcoil sensitivity profiles. SENSE and related methods adopt a spaceperspective they resolve localization ambiguities through algebraicallyextracting additional spatial information encoded with the sensitivityprofiles.

What is needed is a method and system for extended volume imaging, suchas head-to-toe imaging, using a MRI system to reduce the imaging timewhile offering good image quality.

SUMMARY OF INVENTION

In a first aspect, an imaging apparatus for producing Magnetic Resonance(MR) images of a subject is provided. The apparatus comprises a magnetassembly for producing a static magnetic field, a gradient coil assemblyfor generating a magnetic field gradient for use in producing MR images,at least one radiofrequency (rf) coil array disposed about the subjectfor transmitting a radiofrequency pulse and for detecting a plurality ofmagnetic resonance (MR) signals induced from the subject, a positioningdevice for supporting the subject and for translating the subject intothe magnet assembly, and, a plurality of receivers for receiving theplurality of MR signals, the receivers each being adapted to adjusttheir respective center frequencies at a rate commensurate with a rateof translation of the positioning device.

In a second aspect, a method for producing an image from an extendedvolume of interest within a subject using a Magnetic Resonance Imaging(MRI) system where the extended volume of interest is larger than animaging portion of a magnet within the MRI system is provided. Themethod comprises translating the volume using a positioning device alongan axis of the MRI system. A plurality of MR signals are detected fromat least one radiofrequency (RF) coil array for a given field-of-viewwithin the MRI system as the positioning device is translated. Theplurality of MR signals are sent to a plurality of receivers wherein thereceivers are each adapted to adjust their respective center frequenciesat a rate commensurate with a rate of translation of the positioningdevice. A plurality of respective sub-images corresponding to theplurality MR signals for each of the plurality of receivers are combinedto form a composite image of the volume of interest.

BRIEF DESCRIPTION OF DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the invention whenread with the accompanying drawings in which:

FIG. 1 illustrates a simplified block diagram of a Magnetic ResonanceImaging system to which embodiments of the present invention are useful;

FIG. 2 is a side-view block diagram of a receiver arrangement for use inthe MRI system of FIG. 1 and to which embodiments of the presentinvention are applicable;

FIG. 3 is a simplified graphical illustration of a data acquisitionsequence to which embodiments of the present invention are applicable;and,

FIG. 4 is a simplified graphical illustration of a trans-axial view of aRF coil array for use in the MRI system of FIG. 1 and to whichembodiments of the present invention are applicable.

DETAILED DESCRIPTION

FIG. 1 illustrates a simplified block diagram of a system for producingimages in accordance with embodiments of the present invention. In anembodiment, the system is a MR imaging system which incorporates thepresent invention. The MR system could be, for example, a GE-Signa MRscanner available from GE Medical Systems, Inc., which is adapted toperform the method of the present invention, although other systemscould be used as well.

The operation of the MR system is controlled from an operator console100 that includes a keyboard and control panel 102 and a display 104.The console 100 communicates through a link 116 with a separate computersystem 107 that enables an operator to control the production anddisplay of images on the screen 104. The computer system 107 includes anumber of modules that communicate with each other through a backplane.These include an image processor module 106, a CPU module 108, and amemory module 113, known in the art as a frame buffer for storing imagedata arrays. The computer system 107 is linked to a disk storage 111 anda tape drive 112 for storage of image data and programs, and itcommunicates with a separate system control 122 through a high speedserial link 115.

The system control 122 includes a set of modules connected together by abackplane 118. These include a CPU module 119 and a pulse generatormodule 121 that connects to the operator console 100 through a seriallink 125. It is through this link 125 that the system control 122receives commands from the operator that indicate the scan sequence thatis to be performed. The pulse generator module 121 operates the systemcomponents to carry out the desired scan sequence. It produces data thatindicate the timing, strength, and shape of the radio frequency (RF)pulses which are to be produced, and the timing of and length of thedata acquisition window. The pulse generator module 121 connects to aset of gradient amplifiers 127, to indicate the timing and shape of thegradient pulses to be produced during the scan. The pulse generatormodule 121 also receives subject data from a physiological acquisitioncontroller 129 that receives signals from a number of different sensorsconnected to a subject 200, such as ECG signals from electrodes orrespiratory signals from a bellows. And finally, the pulse generatormodule 121 connects to a scan room interface circuit 133 which receivessignals from various sensors associated with the condition of thesubject 200 and the magnet system. It is also through the scan roominterface circuit 133 that a positioning device 134 receives commands tomove the subject 200 to the desired position for the scan.

The gradient wave forms produced by the pulse generator module 121 areapplied to a gradient amplifier system 127 comprised of G_(x), G_(y) andG_(z) amplifiers. Each gradient amplifier excites a correspondinggradient coil in an assembly generally designated 139 to produce themagnetic field gradients used for position encoding acquired signals.The gradient coil assembly 139 forms part of a magnet assembly 141 whichincludes a polarizing magnet 140 and a RF coil 152. Volume 142 is shownas the area within magnet assembly 141 for receiving subject 200 andincludes a patient bore. As used herein, the usable volume of a MRIscanner is defined generally as the volume within volume 142 that is acontiguous area inside the patient bore where homogeneity of main,gradient and RF fields are within known, acceptable ranges for imaging.A transceiver module 150 in the system control 122 produces pulses thatare amplified by an RF amplifier 151 and coupled to the RF coil 152 by atransmit/receive switch 154. In an embodiment of the present invention,transceiver module 150 comprises a plurality of receivers that will bediscussed in greater detail with reference to FIG. 2. The resultingsignals radiated by the excited nuclei in the subject 200 may be sensedby the same RF coil 152 and coupled through the transmit/receive switch154 to a preamplifier 153. The amplified MR signals are demodulated,filtered, and digitized in the receiver section of the transceiver 150.The transmit/receive switch 154 is controlled by a signal from the pulsegenerator module 121 to electrically connect the RF amplifier 151 to thecoil 152 during the transmit mode and to connect the preamplifier 153during the receive mode. The transmit/receive switch 154 also enables aseparate RF coil (for example, a head coil or surface coil) to be usedin either a transmit or receive mode. It is to be appreciated that RFcoil 152 is configured to be operable for MRI scanning as describedbelow, in which a subject is translated on a positioning device alongthe z-axis. As used herein, “adapted to”, “configured” and the likerefer to mechanical or structural connections between elements to allowthe elements to cooperate to provide a described effect; these termsalso refer to operation capabilities of electrical elements such asanalog or digital computers or application specific devices (such as anapplication specific integrated circuit (ASIC)) that is programmed toperform a sequel to provide an output in response to given inputsignals.

As is well-known, RF coil 152 is used for detecting MR signals from theregion of interest that is to be imaged, e.g. whole-body coil, surfacecoil or head coil. To image an extended volume, a whole-body coil isdesirably used for RF coil 152. A whole-body coil may be a conventionaland well-known birdcage coil configuration or, alternatively a varietyof coil structures comprises a plurality of conductors formed in asubstantially circular form interconnected structurally and electricallythat is disposed about a subject to be imaged. The conductors aredesirably of sufficient length to detect signals in a desiredfield-of-view as is also well-known. For embodiments of the presentinvention, RF coil 152 is a receive coil array comprising a plurality ofelements arranged in an array and will be described in greater detailwith reference to FIG. 4.

The MR signals picked up by RF coil 152 are digitized by the transceivermodule 150 and transferred to a memory module 160 in the system control122. When the scan is completed and an entire array of data has beenacquired in the memory module 160, an array processor 161 operates toFourier transform the data into an array of image data. These image dataare conveyed through the serial link 115 to the computer system 107where they are stored in the disk memory 111. In response to commandsreceived from the operator console 100, these image data may be archivedon the tape drive 112, or they may be further processed by the imageprocessor 106 and conveyed to the operator console 100 and presented onthe display 104. As will be discussed with reference to embodimentsbelow, further processing is performed by the image processor 106 thatincludes reconstructing acquired MR image data according to embodimentsdescribed below.

Referring to FIG. 2, there is shown a side-view block diagram of areceiver arrangement for use in the MRI system of FIG. 1. As shown inFIG. 2, subject 200 is positioned on positioning device 134 of the MRscanner. As used herein, MR scanner refers generally to the MRI system.Subject 200 is translated through the useable volume (142 of FIG. 1) ofthe scanner at a constant rate until the entire body of the subject (ordesired portion thereof) has passed through magnet assembly 141 (FIG.1).

Thus, an imaging apparatus for producing Magnetic Resonance (MR) imagesof a subject is provided. The apparatus comprises a magnet assembly forproducing a static magnetic field; a gradient coil assembly forgenerating a magnetic field gradient for use in producing MR images; atleast one radiofrequency (rf) coil assembly for transmitting aradiofrequency pulse and for detecting a plurality of MR signals inducedfrom the subject; a positioning device for supporting the subject andfor translating the subject into the magnet assembly; and, a pluralityof receivers for receiving the plurality of MR signals and being furtheradapted to be adjusted in phase or frequency changes in response totranslation of the positioning device.

Multiple MR sub-images of portions of the subject are made while thesubject is moved through the imaging portion of the MR imaging magnet.MR signals are detected from RF coil 152 and simultaneously sent to aplurality, N, of receivers which are shown as 250, 251 and 252 in FIG.2. The description which follows corresponds to an exemplary embodimentin which N=2, and the receivers are receiver 250 and receiver 251. It isto be appreciated that further embodiments would comprise multiplereceivers and image processing computations described below would beadapted by one skilled in the art for the selected number N ofreceivers. At the completion of subject's 200 movement through magnetassembly 141 (FIG. 1) the sub-images are processed by image processor106 (FIG. 1) and combined to form a composite image of the entiresubject.

The receivers are desirably configured to be adjusted in phase orfrequency in response to phase or frequency changes due to translationof the positioning device. During scanning either the phase or thefrequency of the receivers (or transmitter) is continuously changed tomatch the phase and frequency changes of spin magnetization caused bypatient motion through the geometrically-fixed gradient system of themagnet.

If it is desirable for the frequency-encoding direction to be parallelto the axis of subject motion, then it is the receiver frequency that ischanged at a rate commensurate with the rate of translation of thepositioning device (hereinafter “table motion”). The rate of frequencychange can be determined from the table motion rate (in cm/sec), thegyromagnetic ratio of the nuclear spins (approx. 4250 Hz/Gauss for 1H)and the scan rate (TR) (in ms). The table speed can be assumed to beslow during the short periods during data acquisition itself (typically4–8 ms). In further embodiments, however, the receiver frequency isvaried during data acquisition. If the frequency direction is chosen tobe parallel to the table motion, then the optimal field-of-view (FOV)for the frequency-encoding direction will be the size of the RF coildivided by N.

If the operator desires that the phase-encoding direction of the imageacquisition be parallel to the direction of table motion, then thereceiver phase is changed as a function of an incremented position ofthe positioning device (hereinafter “table position”). In this case theoptimal FOV of the phase-encoding direction of each sub-image would betwice the size of the RF coil to prevent phase-wrap artifacts. If it isdesirable for the slice-selection direction of the image to be parallelto the table motion (as in an axial slice), then the transmitterfrequency is changed to cause the excitation slice to move through themagnet at the same rate that the subject moves. In this case there areno practical restrictions on the FOV of the sub-image.

It should also be noted that oblique scanning is possible by performingsimple matrix rotations to the gradient subsystem and to the receiverand transmitters frequencies in a manner well known to those skilled inthe art.

Additionally, an additional phase or frequency offset is desirably addedto each receiver causing the reconstructed data to be shifted by adistance corresponding to the field-of-view divided by N. In a exemplaryembodiment of the invention in which the frequency-encoding axis of eachsub-image is parallel to the table motion, the acquisition of a singlesub-image is complete in 1/Nth the time it takes the subject to traversethe sensitive imaging volume of the magnet. Furthermore, the order ofdata acquisition can be modified so that k-space is traversed twiceduring the acquisition of each sub-image (e.g. odd numbered rows in thefirst half of the sub-image scan, even numbered rows in the secondhalf). This aspect will be described in greater detail below and withreference to FIG. 3.

Once a sub-image is collected the phase or frequency of the receiver isreset, and a subsequent sub-image at an adjacent location within thesubject is acquired. The process is repeated until the subject's entirebody has passed through the magnet and has been imaged.

Because each receiver operates at a different phase or frequency, imagesacquired with each receiver will contain signals from different portionsof the body. The fields-of-view of each receiver overlap each other,however because the relative offsets are FOV/N. Consequently, every partof the subject's body is acquired in a central portion of one sub-image.

In the exemplary embodiment of the invention, N=2 and 256 lines ofk-space are acquired with the frequency direction of the acquisitionapplied in the direction of table motion. The frequency of bothreceivers is changed at a rate that causes image acquisition to beperformed at the same anatomical location during the acquisition of asub-image. Furthermore, the sub-images acquired with the two receiversare offset from one another by ½ the field-of-view.

With this embodiment the sequence of data acquisition is as follows:

Receiver 1 Receiver 2 Sub image # k-space Sub image # k-space 1 odd — —1 even 1.5 even 2 odd 1.5 odd 2 even 2.5 even 3 odd 2.5 odd 3 even 3.5even —and so on until the entire subject has been imaged.

In this illustrative example which is shown in FIG. 3, the sub-imagenumber represents both the time sequence and the relative location ofthe sub-image in the context of the subject's anatomy.

Note that the same RF pulses and magnetic field gradient pulses are usedto simultaneously generate data from the subject for both receivers.Because there are N=2 passes through k-space, the pulses used to acquirethe second half of sub-image 1 are also used to acquire the first halfof sub-image 1.5.

Once all the data has been acquired, an MR image of the entire body iscreated by combining the central portions of each sub-image. Since eachreceiver operates at a unique offset, every portion of the subject isimaged in a central portion of a sub-image and discontinuity artifactsassociated with image edge boundaries (“stitching artifacts”) areminimized.

Alternatively, the final composite image is created by combining fullsub-images to obtain a composite image with an enhanced signal-to-noiseratio. In this embodiment, each point in the subject's anatomy is foundin N sub images. Since these N images have only partially correlatednoise, combining the full images will provide a signal-to-noiseenhancement. For N=2, the noise in each sub-image is ½ correlated since½ of the data in each sub-image is identical. Thus, the expected SNRgain would be sqrt(3/2) since the data acquired is equivalent to a 1.5NEX acquisition. Conversely, if N=3, then the expected SNR gain would besqrt(5/3) or equivalent to a NEX=5/3 acquisition. In general the SNRgain for using N receivers is given by the expression:Sqrt((2N−1)/N).

As N becomes large, the SNR gain approaches sqrt(2) or that of a NEX=2acquisition (i.e. the SNR gain of imaging twice as long).

A further alternative embodiment employs a single receiver that isoperated with the frequency direction applied parallel to the tablemotion. In this case, the field-of-view of the receiver is set to beequal to a selected active imaging volume less than or equal to theimaging volume. At the beginning of the scan the center of the sub-imageis set to be ¼^(th) of the way into the active volume of the imagingsystem. Sub-images are collected over a period of time that correspondsto the time it takes a selected portion of anatomy to traverse ½ of theactive imaging volume. Thus at the end of the acquisition of asub-image, the center of the sub-image is ¾^(th) of the way into theactive volume of the imaging system. Upon the completion of thesub-image the next sub-image is initiated with an offset equal to 2 theFOV. Once all the sub-images have been collected the central portion ofeach sub-image is extracted and combined with other central portions toform the composite image. As with the prior embodiment, extraction ofthe central portions of the image reduces the severity of “stitching”artifacts.

In another further embodiment, a single receiver is employed wherein thereceiver is adapted to receive the plurality of MR signals from rf coil152 and is further adapted to be adjusted in at least one of phase andfrequency in response to translation the positioning device through themagnet assembly. In this embodiment, the receiver is also furtheradapted to collect image data for a field-of-view corresponding to auseable volume of the magnet assembly. Thereafter, processing of the MRsignals is performed to compute a plurality of respective sub-images forthe field-of-view (FOV) at each of a plurality of incremented positionsas the subject is translated. A central portion of each of the pluralityof respective sub-images is combined to form a composite image of thevolume of interest.

Referring to FIG. 4, an embodiment for the present invention employs aseparate RF receive coil array 353 that complements RF coil 152 (of FIG.1). During moving table imaging, RF coil 152 is used for transmissionand coil array 353 is used for reception. Arranging coil array elements300 of coil array 353 (combination of elements 300 comprises the receivecoil array) such that they distribute in dimensions orthogonal relativeto the frequency encoding direction is desirable for reducing phaseencoding steps and speeding up the imaging. For moving table imagingmethods that frequency-encode along the table translation direction, apreferred embodiment employs coil array 353 of coil array elements 300that are placed on a fixture 310 that wraps around the examined subject200 (FIG. 1) but remains stationary with respect to the scanner (seeFIG. 4). Alternatively, the array of receive coils 353 do not remainstationary and are moved concurrently with the table motion. In eitherembodiment, during moving table imaging, the coils in the array receivein parallel. A moving table imaging reconstruction algorithm thatintegrates the parallel imaging reconstruction concept is used toreconstruct full-FOV images. It is to be appreciated that other rf coilarray configuration, as are well-known in the art, may be employed withmethods of the present invention. Other such configurations may includearranging coil array elements 300 of receive coil array 353 such thatthey wrap around only the anterior portion of subject 200, and/oradditionally distribute along the frequency encoding direction.

Unlike a body coil, the B1 field of any of the receive coil arrayelements may vary spatially and induce image artifacts. Addressing theartifacts will be described in greater detail below. The methods ofimaging (acquisition and reconstruction) as described earlier with amoving table are applicable when employing a receive coil array ratherthan a body coil.

In embodiments employing receive coil arrays, data acquisition isconducted concurrently with table translation. The receivers' centerfrequencies are adjusted, or alternatively swept, at a rate commensuratewith the table motion, which allows the scanner to follow a selected setof spins during each round of phase encodes. This enables each of thereceivers, or alternatively channels, to acquire a coherent k-space datamatrix and produce a regional image that maps out the set withnegligible effects of the concurrent translation. Reduction of k-spacesampling density is in effect in the present case, and leads to aliasingalong the phase encoding direction(s) in each of the regional images.Applying SENSE or other parallel imaging reconstruction on the regionalimages that are produced in parallel generates a regional image free ofaliasing. The process is repeated to produce a series of regional imagesof the volume of interest along the translation direction, which can becombined into a full-FOV image. In the presence of B1 fieldinhomogeneity however, the followed set of spins is “seen” by each coilwith a different sensitivity weighting at different table locations,especially for embodiments that employ a stationary coil array 153 toreceive signals during table translation. This leads to undesirableview-to-view changes that may cause significant ghosting artifacts. Twostrategies may be applied to alleviate/eliminate this problem. First,one may design the coil array elements 300 to assume shapes that extendthe full travel range of the selected spins, which is determined by thebandwidth of the signal filtration during each view acquisition, tabletranslation speed, and pulse sequence timing. This reduces B1 fieldvariation along the translation direction and hence lessens view-to-viewchanges. Second, one may, in the reconstruction, take into account boththe B1 field maps and the table locations and correct for theview-to-view changes algebraically.

Embodiments of the present invention provide for a MRI system that maybe employed as a whole body screening tool for metastatic cancer andother diseases. In the embodiments for the methods of extended volumeimaging, various two-dimensional or three-dimensional MR pulse sequences(e.g. spin echo, fast spin echo, echo-planar, gradient echo, and FIESTAor FISP) may be employed. The imaging sequence may be one ofmulti-slice, multi-slab, and volume imaging sequences. Use of many MRpulse sequence is possible since only the phase or the frequency of thereceiver and/or transmitter is modified during a scan. Embodiments ofthe methods of the present invention allow the user to select thedesired direction of the phase and frequency encoding directions inorder to advantageously place phase-encoding artifacts in desireddirections. The user is also able to select interleaved acquisition ofmultiple slices, if necessary.

In further embodiments, embodiments of the present invention areimplemented as a hardware subsystem in the scanner that is independentof the pulse sequence itself. That way any pre-existing imaging strategyand information content can be obtained over the entire subject. Ahardware system implementation would comprise incorporating a subsystemin the scanner that varies the receiver offset frequency or phase inresponse to changes in table position, or alternatively, changes thetable position in response to changes in receiver frequency/phase.

While the preferred embodiments of the present invention have been shownand described herein, it will be obvious that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those of skill in the art without departingfrom the invention herein. Accordingly, it is intended that theinvention be limited only by the spirit and scope of the appendedclaims.

1. A method of producing an image from an extended volume of interestwithin a subject using a Magnetic Resonance Imaging (MRI) system, themethod comprising: translating the extended volume of interest along anaxis of the MRI system using a positioning device and imaging portionsof the extended volume of interest when they are within the imagingportion of an imaging magnet within the MRI system, wherein the extendedvolume of interest is larger than an imaging portion of the imagingmagnet; detecting a plurality of MR signals from at least oneradiofrequency (RF) coil array for a given field-of-view within the MRIsystem as the positioning device is translating the volume; sending theplurality of MR signals to a plurality of receivers, the receivers eachbeing configured to adjust a receiver parameter; wherein the receiverparameter is adjusted based on direction of the image parallel to amotion of the subject; computing a plurality of respective sub-imagescorresponding to the plurality MR signals for each of the plurality ofreceivers and for the given field-of-view (FOV) at a plurality ofincremented locations of the subject; extracting a central portion fromeach of the respective sub-images; and combining only the extractedcentral portion of each of the respective sub-images in order to form acomposite image of the extended volume of interest.
 2. The method ofclaim 1 wherein the at least one rf coil array is mounted on a fixturethat is disposed about the subject.
 3. The method of claim 2 wherein thefixture and rf coil array mounted thereon are stationary relative to thestatic magnetic field.
 4. The method of claim 2 wherein the fixture andrf coil array mounted thereon are moveable relative to the staticmagnetic field.
 5. The method of claim 1 wherein the at least one rfcoil array comprises a plurality of coil elements arranged in aorthogonal distribution relative to a frequency encoding direction. 6.The method of claim 1 wherein the detecting step is performedconcurrently with the translating step.
 7. The method of claim 1 whereinthe translating step is repeated until a selected length of the subjecthas been imaged inside the imaging portion of the magnet.
 8. The methodof claim 1 wherein the extended volume of interest is a head-to-toe viewof the subject.
 9. The method of claim 1, wherein the receiver parametercomprises a receiver frequency, and wherein the receiver frequency isadjusted in response to a translation of the positioning device; whereinthe receiver frequency is adjusted when a frequency encoding directionof the image is parallel to an axis of a motion of the subject.
 10. Themethod of claim 1, wherein the receiver parameter comprises a receiverphase, and wherein the receiver phase is adjusted in response to atranslation of the positioning device; and wherein the receiver phase isadjusted when a phase encoding direction of the image is parallel to anaxis of a motion of the subject.
 11. The method of claim 1, wherein therf coil array is configured to adjust a transmit frequency in responseto a translation of the positioning device; and wherein the transmitfrequency is adjusted when a slice selection direction of the image isparallel to an axis of a motion of the subject.
 12. The method of claim1, wherein the computing of the sub-images acquired from each receiveris offset by a fraction of the field of view, wherein the fraction ofthe field of view equals the field of view divided by a number ofreceivers.
 13. A method for imaging an extended volume of interestwithin a subject using a Magnetic Resonance Imaging (MRI) systemcomprising: translating the subject into an imaging portion of a magnetassembly of the MRI system; detecting a plurality of MR signals from aradiofrequency (RF) coil array; sending the plurality of MR signals to aplurality of receivers, the receivers each being configured to adjust areceiver parameter, wherein the receiver parameter is adjusted based ondirection of the image parallel to a motion of the subject;reconstructing at least one image of the extended volume of interest bycomputing a plurality of respective sub-images corresponding to theplurality MR signals for each of the plurality of receivers and for thegiven field-of-view (FOV) at a plurality of incremented locations of thesubject as the subject is translated; extracting a central portion fromeach of the respective sub-images; and combining only the centralportion of each of the plurality of the respective sub-images in orderto form a composite image of the extended volume of interest.
 14. Themethod of claim 13 wherein the extended volume of interest is ahead-to-toe view of the subject.
 15. The method of claim 13 wherein theat least one rf coil array comprises a plurality of coil elementsarranged in orthogonal distribution to a frequency encoding direction.16. The method of claim 13 wherein the at least one rf coil array ismounted on a fixture that is disposed about the subject.
 17. The methodof claim 16 wherein the fixture and rf coil array mounted thereon arestationary relative to the static magnetic field.
 18. The method ofclaim 16 wherein the fixture and rf coil array mounted thereon aremoveable relative to the static magnetic field.
 19. The method of claim13 wherein the detecting step is performed concurrently with thetranslating step.
 20. The method of claim 13 wherein the translatingstep is repeated until a selected length of the subject has been imaged.21. The method of claim 13 wherein the translating step is substantiallycontinuous.
 22. The method of claim 13, wherein the receiver parametercomprises a receiver frequency, and wherein the receiver frequency isadjusted in response to a translation of the positioning device; whereinthe receiver frequency is adjusted when a frequency encoding directionof the image is parallel to an axis of a motion of the subject.
 23. Themethod of claim 13, wherein the receiver parameter comprises a receiverphase, and wherein the receiver phase is adjusted in response to atranslation of the positioning device; and wherein the receiver phase isadjusted when a phase encoding direction of the image is parallel to anaxis of a motion of the subject.
 24. The method of claim 13, wherein therf coil array is configured to adjust a transmit frequency in responseto a translation of the positioning device; and wherein the transmitfrequency is adjusted when a slice selection direction of the image isparallel to an axis of a motion of the subject.
 25. The method of claim13, wherein the computing of the sub-images acquired from each receiveris offset by a fraction of the field of view, wherein the fraction ofthe field of view equals the field of view divided by a number ofreceivers.